1. Field of the Invention
The present invention relates to a noise filter provided to a ground line for discharging a short-circuit current generated on an electronic apparatus to the ground, and in detail, to a noise filter having an inductor which suppresses noise induced on the ground line.
2. Description of the Related Art
A conventional noise filter of this type comprises an inductor, having a function to discriminate a frequency of an undesired signal called “noise” from others to thereby pass a short-circuit current from the commercial power supply to the ground (for example, see the Japanese Utility Model Laid-open No. 61-140620 (FIG. 1, etc.)). The noise has a frequency of, for example, 10 kHz or higher. The commercial power supply has a frequency of 50 Hz or 60 Hz in Japan.
FIG. 10(a) is a circuit diagram showing a conventional noise filter. FIG. 10(b) is a circuit diagram showing a state where the noise filter shown in FIG. 10(a) is used. The following will describe the noise filter with reference to these Figures.
A noise filter 70 is a two-terminal type noise filter for use on a ground line, comprising one inductor 71. One terminal 72 is grounded through a ground line 74 and the other terminal 73 connects to an electronic apparatus 75. Further, inductance of the inductor 71 is so set that a short-circuit current Is of the commercial power supply caused by a leakage or the like is passed from the electronic apparatus 75 to the ground 76, and also a noise current In induced on the ground line 74 may be blocked.
However, the conventional noise filters have the following problems.
(1) A noise power referred to as “noise” is induced not only in a steady-state current on the ground line 74 but also in a pulse-state current with non-periodically. In such a case, the inductor 71 constituting the noise filter 70, which charges power thereon owing to magnetic and electric fields, discharges the charged power when noise power stops flowing thereto. As such, this discharged power may cause the electronic apparatus 75 to fail or temporarily deteriorate in functioning.
(2) In order to reduce the noise current In induced on the ground line 74, the inductor 71 is effective as described above. On the other hand, large electronic apparatuses, among the electronic apparatuses 75 connected to the ground line 74, have considerably large earth capacitance C, which may sometimes be combined with the inductor 71 to give rise to series resonance. When the resultant resonance frequency current flows on the electronic apparatus 75, a failure due to noise arises.
(3) It is preferable that the inductance of the inductor 71 be as high as possible in order to block the noise current In, but that the inductance be as low as possible in order to conduct the short-circuit current Is. It has been considerably difficult to do a balancing act between these contradicting characteristics. That is, when the inductance is increased in order to fully block the noise current In, the short-circuit current Is is not well conducted, and when the inductance is decreased in order to well conduct the short-circuit current Is, the noise current cannot be fully blocked.